The present invention generally relates to fabrication of semiconductor devices and, more particularly, to an electron-beam exposure system that exposes a semiconductor pattern on an object such as a semiconductor substrate by means of a charged particle beam, such as an electron-beam.
Electron beam lithography is an indispensable technology for fabricating leading edge semiconductor devices having a very large integration density. By using electron-beam lithography, it is possible to expose a pattern having a width of 0.05 .mu.m or less with an alignment error of 0.02 .mu.m or less. Thus, it is expected that electron-beam lithography will play a central role in the fabrication of future semiconductor devices, such as a large capacity DRAM having a storage capacity exceeding 256 Mbits or 1 Gbits, or a high speed microprocessor having extremely powerful arithmetic functions.
FIG. 1 shows a schematic view of a conventional electron-beam exposure system using a blanking aperture array. Referring to FIG. 1, the electron-beam exposure system is generally formed of an electron optical system 100 for producing a focused electron-beam and a control system 200 that controls the electron optical system 100. The electron optical system 100 includes an electron gun 101 as an electron-beam source, and the electron gun 101 emits an electron-beam in the form of a divergent beam along a predetermined optical axis O.
The electron-beam thus produced by the electron gun 101 is shaped by an aperture 102a provided on an aperture plate 102, wherein the aperture 102a is provided in alignment with the optical axis O and shapes the electron-beam incident there, so as to have a rectangular cross section.
The shaped electron-beam thus formed is focused upon the BAA (blanking aperture array) mask 110 by an electron lens 103, wherein the BAA mask 110 carries thereon a blanking aperture array. Thus, the electron lens 103 projects an image of the aforementioned rectangular aperture 102a upon the BAA mask 110. The BAA mask 110 carries a plurality of small apertures corresponding to exposure dots to be formed on a semiconductor substrate 115 in the form of BAA, and an electrostatic deflector is provided on the BAA mask 110 in correspondence to each of such apertures. The electrostatic deflector is controlled by a driving signal E such that an aperture cooperating with the electrostatic deflector passes the electron-beam directly therethrough in a non-activated state of the electrostatic deflector, while in the activated state, the electrostatic deflector deflects the electron-beam passing through the aperture away from the optical axis O. As a result, there is formed an exposure dot pattern on the semiconductor substrate 115 in correspondence to the non-activated apertures of the BAA mask 110.
The electron-beam, having passed through the BAA mask 110, is focused at a focal point f.sub.1 on the optical axis O after passing through the electron lenses 104 and 105 that form a demagnifying optical system, and the image of the selected apertures is formed at the focal point f.sub.1. The focused electron-beam is further focused on the semiconductor substrate 115, held on a movable stage 114, by electron lenses 106 and 107 that form another demagnifying optical system, after passing through a round aperture 113a provided on a round aperture plate 113. Thus, an image of the BAA mask 110 is projected on the substrate 115. Here, the electron lens 107 acts as an objective lens and includes therein various correction coils 108 and 109 for correcting the focal point and aberrations as well as deflectors 111 and 112 for moving the focused electron-beam over the surface of the substrate 115.
Between the lens 104 and lens 105, there is provided an electrostatic deflector 116, wherein the electrostatic deflector 116 deviates, upon activation, the path of the electron-beam from the optical axis O, which is set to pass through the round aperture 113a on the plate 113. As a result, it becomes possible to switch the electron-beam on and off at a high speed on the semiconductor substrate 115 by energizing the electrostatic deflector 116. Further, it should also be noted that the electron beams on the BAA mask 110, described above, deviate also from the round aperture 113a. Therefore, the electron beams thus deflected do not reach the semiconductor substrate, and it becomes possible to control the exposure dot pattern on the substrate 115 in response to the energization of the BAA mask 110.
In order to control such an exposure operation, the electron-beam exposure system of FIG. 1 uses the aforementioned control system 200. The control system 200 includes an external storage device 201, such as a magnetic disk device or a magnetic tape device, for storing data relating to the patterns of the semiconductor device to be exposed.
The data stored in the storage device 201 is read out by a CPU 202, and the data compression thereof is removed by a data expansion circuit 203 that includes a buffer memory 203a, a data expansion unit 203b and a canvas memory 203c. As a result of the expansion, exposure data is converted to exposure dot data that switches the individual apertures on the BAA mask 110 on and off according to the desired exposure pattern.
In order to achieve a delicate correction of the exposure pattern, the electron-beam exposure system of FIG. 1 conducts a multiple exposure of the exposure dots on the substrate 115, such that the exposure is superposed N times, each time with an independent exposure pattern. For this purpose, the data expansion circuit 203 includes N identical circuits 203.sub.1 to 203.sub.N, wherein each of the circuits 203.sub.1 to 203.sub.N produces, based upon the exposure data provided from the CPU 202, N sets of mutually independent exposure dot pattern data used for carrying out the foregoing multiple exposures to be superposed N times. The dot pattern thus obtained is then supplied to the BAA mask 110 via driving circuits 204 and 205 provided in correspondence to each of the apertures on the BAA mask 110, for driving the electrostatic deflector provided on each of the apertures on the BAA mask 110. Thereby, the BAA mask 110 shapes the electron-beam incident thereto and produces an electron-beam bundle that forms the desired exposure dots according to the desired exposure pattern.
The electron-beam thus shaped is deflected by the main deflector 111 and the sub-deflector 112 respectively driven by a main deflection control circuit 207 and a sub-deflection control circuit 208 under control of an exposure control unit 206, such that the electron-beam scans over the surface of the substrate 115. It should be noted that the main deflection control circuit 207 is supplied with main deflection data from the exposure control unit 206 and produces a drive current signal that drives the main deflector 111. On the other hand, the sub-deflection control circuit 208 is supplied with sub-deflection data from the exposure control unit 206 and produces a drive voltage signal that drives the sub-deflector 112.
The main deflection control circuit 207 further includes a distortion correction circuit 207a, wherein the distortion correction circuit 207a compensates for the astigmatism by driving the correction coil 109 via an astigmatic correction circuit 207b. Further, the correction circuit 207a achieves the focusing compensation by driving the focusing correction coil 108 via a focusing correction circuit 207c.
In addition, the system of FIG. 1 is equipped with a refocus control circuit 203e for compensating for the divergence of the focused electron-beam as a result of Coulomb repulsion, wherein the refocus control circuit 203e adjusts the strength of the electron lens 106 in response the exposure dot pattern.
FIG. 2A shows the function of the main deflector 111 and the sub-deflector 112 in the electron-beam exposure system of FIG. 1. In FIG. 2A, it should be noted that the electron optical system of FIG. 1, that is located at the upstream side of the objective lens 107, is omitted from the illustration for the sake of simplicity.
Referring to FIG. 2A, the electron-beam incident to the objective lens 107 is deflected by the main and sub-deflectors 111 and 112 and moves over the surface of the substrate 115. It should be noted that the main deflector 111 is formed of an electromagnetic deflector as usual and covers a relatively large main deflection area A having a size of about 5 mm.times.5 mm as indicated in FIG. 2B. On the other hand, the sub-deflector 112 is formed of an electrostatic deflector and covers a relatively small sub-deflection area typically having a size of 100 .mu.m.times.100 .mu.m. It should be noted that FIG. 2B shows the exposure achieved on the surface of the substrate 115.
In operation, the electromagnetic deflector is energized to deflect the electron-beam over the substrate 115, such that a desired sub-deflection area is selected from a main deflection area in which a number of sub-deflection areas are included. Next, the sub-deflector 112 is energized such that the electron-beam is moved over the selected sub-deflection area at a high speed, and the exposure pattern is formed as a result of such a scanning of the electron-beam. Generally, the electromagnetic deflector exhibits a slow response, due to the inductance of the coil forming the deflector, while the operation of the electrostatic deflector is very fast.
It should be noted that such construction and operation of the main and sub-deflectors are more or less the same in other general electron-beam exposure systems. A more complete description of the electron-beam exposure system of FIG. 1 can be seen in the U.S. patent application Ser. No. 08/241,409, which is incorporated herein as reference.
FIGS. 3A and 3B show the construction of the conventional drive circuit that drives the sub-deflector 112 of FIG. 2A, wherein it should be noted that the circuit of FIG. 3A corresponds to the sub-deflection control circuit 208 of FIG. 1.
Referring to FIG. 3A, the sub-deflector 112 is formed of an X-deflection system that deflects the electron-beam in an X-direction and a Y-deflection system that deflects the electron-beam in a Y-direction that is perpendicular to the X-direction. The X-deflection system includes a D/A converter 208X.sub.1 supplied with sub-deflection data DX indicative of the magnitude of deflection of the electron-beam in the X-direction, for converting the same to an analog signal Ix, and a D/A converter 208X.sub.2 supplied with sub-deflection correction data D.sub..DELTA. X indicative of the deviation of the electron-beam due to the vibration of the stage or vortex current or the effect of the ECC correction, for converting the same to an analog signal .sub..DELTA. Ix. The X-deflection system further includes an amplifier 208X.sub.3 supplied with the analog signals Ix and .sub..DELTA. Ix for producing a sum signal V.sub.x +.sub..DELTA. V.sub.x, an output amplifier 208X.sub.4 for producing a drive voltage signal G(V.sub.x +.sub..DELTA. V.sub.x) to be supplied to a first electrode plate of the electrostatic deflector by amplifying the foregoing sum signal with a gain G, and another output amplifier 208X.sub.5 that amplifies the foregoing sum signal with a gain -G to produce a drive voltage signal -G(V.sub.x +.sub..DELTA. V.sub.x) that is supplied to a second, opposing electrode plate of the electrostatic deflector. Thereby voltages G(V.sub.x +.sub..DELTA. V.sub.x) and -G(V.sub.x +.sub..DELTA. V.sub.x) are applied across a pair of electrodes opposite with each other in the X-direction. Further, a similar construction is provided for the Y-deflection system.
FIG. 3B shows the actual construction of an amplifier 208 corresponding to each of the amplifiers 208X.sub.4 and 208X.sub.5 or 208Y.sub.4 and 208Y.sub.5. As will be noted from FIG. 3B, the amplifier is formed of an operational amplifier having a feedback loop that provides a stabilized gain determined by the resistances Rs and Rf.
When the electron-beam is deflected by the main deflector in the electron optical system of FIG. 2A, the electron-beam inevitably deviates from the optical axis O. Thereby, there arises a problem of astigmatism in the electron-beam in that the electron-beam spot on the substrate 115 tends to be distorted as a result of such astigmatism. Associated with such a distortion of the electron-beam spot, there may also occur a change in the size of the beam spot. While such an astigmatism may be compensated for by the compensation coil 109 shown in FIG. 2A, there still occurs a problem of distortion of the electron-beam, when the beam deflected by the main deflector 111 is further deflected by the sub-deflector 112, due to a non-uniform electric field inside the electrostatic deflector. It should be noted that the electron-beam thus deflected passes in the vicinity of the electrode plate of the electrostatic deflector where the distribution of the electric field is not uniform.
FIGS. 4A and 4B explain the astigmatism of the electron-beam in a sub-deflection region 115A caused as a result of such an electrostatic deflector, wherein FIG. 4A shows a case in which the sub-deflection region selected by the main deflector 111 is located on or in the vicinity of the optical axis, while FIG. 4B shows a case in which the selected sub-deflection region is located close to the edge of the main deflection region.
Referring to FIG. 4A where the center of the sub-deflection region 115A is coincident to the optical axis O, it will be noted that no distortion occurs in the electron-beam as long as the electron-beam travels along the optical axis O. Further, because of the small magnitude of beam deflection in the sub-deflection region 115A caused by the main deflector, the distortion of the electron-beam is generally small, while it will be noted that there is a slight distortion of the electron-beam in the marginal part of the sub-deflection region 115A.
In the situation of FIG. 4B where the electron-beam is deflected significantly by the main deflector, it should be noted that electron-beam on the central part of the sub-deflection region 115A is still free from distortion due to the compensation by the correction coil 109, while the electron-beam deflected to the marginal region of the sub-deflection region 115A experiences a heavy distortion.
FIG. 5 shows the distribution of potential in such a sub-deflection region. As will be noted from FIG. 5, the electric field potential, which should be distributed uniformly with a uniform gradient, is distorted significantly at the part close to the edge of the electrostatic deflector. In FIG. 5, the electrode plates are provided generally coincident to the X- and Y-coordinates. Such a distortion of the electric field arises from various factors such as the electrode plates of the electrostatic deflector being located at a finite distance from the electron-beam rather than at an infinite distance, the number of the electrode plates around the electron-beam being finite rather than infinite, and the like. Thus, when the electron-beam is offset from the optical axis O, the electron-beam passes the neighborhood of the electrode plate of the electrostatic deflector and experiences the effect of the distorted electric field strongly.
In principle, the distortion of the electron-beam shown in FIGS. 4A and 4B can be eliminated by driving the compensation coil 109, provided for astigmatic correction, in response to the deflection of the electron-beam. However, the inductance of the compensation coil 109 does not allow effective control of the drive current supplied to the compensation coil 109 in response to the high speed deflection of the electron-beam inside the sub-deflection region 115A. The response of the correction coil is too slow for responding to the high speed deflection by the electrostatic deflector. Further, such a use of the correction coil 109 for elimination of the beam distortion shown in FIGS. 4A and 4B is discouraged, as it is necessary to map the drive current, not only in each of the sub-deflection regions, but also in response to each exposure dot in each of the sub-deflection regions. For this purpose, an enormous memory is needed.